In recent years, techniques to melt-knead functional additives have been investigated for the purpose of imparting various functions to various kinds of thermoplastic resins. Many of those functional additives are inorganic fillers or organic additives having a significantly high melting point or decomposition temperature and thus these do not melt at a normal extrusion temperature. It is desirable for such additives which do not melt at the time of extrusion to be in a high dispersion state in order to exert higher performance, and thus the shape thereof tends to be miniaturized, that is, finely pulverized.
A twin screw extruder can favorably mix and disperse many materials and discharge in a stable extrusion amount, and thus is widely used in the manufacture of a resin kneaded product, or the like. However, there are many cases in which the amount of the material entering into the supply section of a twin screw extruder (the amount that enters into a twin screw extruder) limits the extrusion amount when a material containing a large amount of fine powder with a small bulk specific gravity or the like is extruded.
As a method for solving this, a method using a forced supply device (compactor) can be exemplified. However, the effect thereof is not sufficient in the case of a fine powder that is a significantly fine particle, has a small bulk specific gravity, and is easily fluidized although a compactor is used, and thus it is difficult to obtain a desired extrusion amount. As a result, the operation can only be performed in a limited range of extrusion conditions.
The major factor that prevents the entering of the material containing a fine powder into the supply section is a large amount of air contained in the fine powder due to the small bulk specific gravity thereof. This material containing a large amount of air is compressed in a twin screw extruder, air is separated, and the air flows to the supply section side of the twin screw extruder, that is, in a direction opposite to the moving direction of the material. As a result, the fine powder is in a fluidized state within the barrel. For this reason, the apparent friction coefficient of the material decreases, the compression of the fine powder by the screw hardly proceeds, and the original mass transport amount of the screw is reduced as a result. The extrusion action is not lost even in this case since the material is softened by the heat from the barrel and the compression melting proceeds in combination with the shear force caused by the screw. However, the extruder can only exert the ability which is much lower compared to the extrusion amount obtained by compressing and melting the material primarily by the mechanical shear heat which is the original function of a twin screw extruder. As described above, it is likely to occur a phenomenon, so-called feed-neck phenomenon, that the material at the supply port does not enter into the inside of the extruder since the air remains in the inside of the twin screw extruder.
To cope with such a problem, a method is suggested in, for example, Patent Document 1 in which the relative velocity of the fine powder and the air is reduced by not allowing the air contained in the fine powder to flow backward to the supply port of the extruder but releasing the air through the opening provided downstream the supply port and thus the material is extruded without being in the fluidized state.